This invention relates to a means and method of carrying out the calibration of gas monitors.
There are today a great variety of gas monitors used particularly in the medical field in order to continuously monitor gases to and from a patient for various reasons.
In particular, during anesthesia, the gas monitors that are normally used include O.sub.2 monitors, CO.sub.2 monitors and also agent monitors that display the percentages of anesthetic vapor in the gases being delivered to the patient to carry out anesthesia. Such anesthetic agents include halothane, enflurane, isoflurane and are often administered concurrently with nitrous oxide, particularly during the induction period.
Such monitors are, of course, relied upon heavily in carrying out operations on the patient and therefore their accuracy must be assured. The gas monitors, however, exhibit certain drift over a period of time and therefore are recalibrated on a regularly timed sequence, that is every so many hours of use. To carry out that calibration, a calibration gas is utilized and which contains a precise known amount of the gas detected by the monitor.
As an example, a halothane agent monitor may be calibrated with a typical calibration gas comprised of 2.0 (%) percent halothane in nitrogen commercially available in 0.5 liter containers at a pressure of about 80 psig. Larger calibration gas containers are also readily available, however, for medical instruments, only small containers are generally needed. By passing that calibration gas through the instrument that monitors halothane, the operator can determine whether the monitor read-out displays the accurate percentages of halothane, and, if not, the monitor is corrected to that desired reading.
In current calibration practices, the calibration gas is introduced into an inlet in the gas monitor in a manner deliberately designed for leakage, that is, a path to ambient atmosphere is allowed in order to avoid over pressuring the gas monitor. Calibration gas is readily provided in the aforementioned pressurized containers and can harm the gas monitor if that full pressure is allowed to act upon the gas monitor sensor or internal pumping mechanism. After the calibration gas passes through the gas monitor, it is generally released to ambient atmosphere and is thus dissipated in the same surrounding atmosphere as is breathed by the personnel carrying out the calibration.
One of the drawbacks of such system, therefore is the potential hazard due to the pollution of the atmosphere in which the personnel work. The hazards of amounts of certain agents such as halothane and nitrous oxide are well known and therefore any environment in which these gases are present puts the personnel at some risk.
In addition, by merely passing the calibration gas through the gas monitor, the volume that the gas monitor actually requires to carry out an accurate calibration is probably less than is recommended to be used, thus, the excess calibration gas is wasted. Since there is no accurate determination made of the calibration gas through the gas monitor, the operator uses some judgment in the amount of calibration gas and it is assumed that such operator would prefer to err on the side of excess, that is, they will administer more calibration gas than is technically needed in order to be sure that enough was used.
One drawback with the use of calibration gases in commercially available containers is the presence, in such containers, of flow restrictors, built into the pressurized containers to reduce the flow of calibration gas therefrom. As the pressure in the container is reduced through normal use, the flow from the container is also reduced to the extent that the actual flow is not known to the operator. For example, a new container may initially provide a flow of 1.5 liters/minute but as the container is used, the flow could easily be reduced to 50 ml./min or lower. If therefore, the instrument requires 300 ml./min for accurate calibration, the pump in the gas monitor using present calibration methods will draw in atmospheric air to supplement the difference, thus causing an inaccurate calibration. By conventional means it is not possible for the operator to know precisely when the flow from the calibration gas containers is insufficient for the requirements of the instrument.
The present methods and means are therefore both wasteful of expensive calibration gas and also create potentially hazardous working conditions for the personnel that carry out the calibration of such gas monitors.